In GERAN (GSM Edge Radio Access Network), measurement reports (i.e. messages containing signal strength/quality measurements of neighbouring cells) are sent in fixed-size messages. Each message is approximately 20 octets in length, and typically contain information for around 6 cells.
Some measurement reports in GERAN do not explicitly indicate the RAT (radio access technology) of the measured cell: instead, frequency indices are used and the range (0-31) is split into GSM (Global System for Mobile Communications), UMTS (Universal Mobile Telecommunications System), E-UTRAN (Evolved UMTS Terrestrial Radio Access Network).
The network can signal measurement reporting criteria (e.g. minimum signal strength and/or quality); if a cell does not fulfil these criteria, the mobile shall not report that cell in a measurement report. If more cells meet the criteria than can be included in a single report, a prioritisation scheme is specified (some parameters of this prioritisation may be signalled by the network).
Some cells are uncontrolled in the sense that a PLMN (public land mobile network) operator does not control the location/operation of the cell. Other cells are controlled in the sense that the operator does control the location/operation of the cells. Examples of uncontrolled cells include, for example, CSG (closed subscriber group) cells, cells controlled by home enode b's and home node b's. These cells are also sometimes referred to as being “uncoordinated” in the sense that they are not subject to normal radio/cell planning.
It is generally understood that there are many methods of performing a handover for controlled cells. Handover in this context refers to a cell change where resources (for example timeslots, frequency channels, scrambling codes, etc.) for transmission and/or reception in a target cell are allocated to a mobile station in advance of the mobile station performing a cell change to that cell, particularly in response to a request from the controller of the device's serving cell.
While the uncontrolled cells may be configured to use spectrum that is owned by the operator, the network operator does not have the same control over uncontrolled cells as for controlled cells. Typically, the network operator does not own the support structure (towers etc.), does not own or control the backhaul connection (e.g. Digital Subscriber Line (DSL) connections), does not know or control when a given uncontrolled cell is going to be switched on, and/or may not know or control the locations of uncontrolled cells. The operator will typically still have control of various parameters such as operating frequency, transmit power, etc. if the operator owns the spectrum license.
In order to perform a handover to a target cell, controlled or uncontrolled, a MS (mobile station) typically needs to provide identifying information relating to the target cell to the current serving cell so that it can initiate handover process. In particular, the current serving cell needs to be able to communicate with (possibly via a core network) the cell controller for the target cell. However, the current serving cell may not be aware of how to reach the cell controller for a target uncontrolled cell, unless provided with explicit identification information for the cell (such as a cell global identity). This makes handover to such cells difficult. In contrast, for a controlled target cell, it may be sufficient for the mobile station to provide the current serving cell with information about the target cell which does not require or cause the acquisition of any broadcast information from the target controlled cell for the serving cell to be able to reach the cell controller for the target controlled cell, since the serving cell or some part of the network may be able to map other identifying features of the cell (such as operating frequency, scrambling code etc.) to the identity of the target cell or its controller.
A cell controller may not necessarily map physical layer identities (such as frequency, spreading code, etc.) of an uncontrolled cell (more generally, another cell) to the identity of its respective controller (or to an identifier which can be used elsewhere in the core network to route messages to the target controller).
In general therefore, it is expected that, for handover to an uncontrolled cell, the mobile station will have to provide more identification information regarding the target cell than in the case of handover to a controlled cell.
Identification of CSG cells in measurement reports is accomplished through the inclusion of “routing parameters”. These routing parameters are used by the serving cell controller to contact the target cell's controller and reserve resources and request a handover command message. The MS typically obtains these in advance of sending the measurement report by reading the broadcast system information of the target cell; it may also have this information stored.
Some existing measurement report messages identify cells (in part) using a “frequency index” (0 . . . 31), each frequency index representing a regular (i.e. non-CSG) physical carrier frequency. An index may be used for (at most one of) GSM, UMTS or E-UTRAN frequencies. The mapping of index to physical frequency is based on complex rules for processing neighbour cell lists (NCLs) received by the mobile station.